Field of the Invention
This invention relates to a method for making particles. More particularly, it relates to a method of making uniformly-sized polymeric particles, with a very low population of oversized particles.
It is known that particles can be made by a variety of in situ polymerization techniques such as suspension polymerization or limited coalescence (LC) polymerization as well as crushing or grinding of solid polymer samples, spray drying, evaporation of solvent from a dispersion of a polymer solution, etc. In order to obtain a narrow particle size distribution of useful product, all of these techniques require extensive sorting and classification operations that are costly, time-consuming and cause substantial reductions in yield. LC polymerization frequently provides nearly monodisperse particle size distributions. However, even with LC polymerization, the particle size distribution of polymer particles is not ideal. In particular, it is commonly observed that a substantial population of oversized particles is formed. Further, particularly when larger particles are being made, an appreciable amount of coagulum results. The presence of these materials introduces the necessity for complex processing steps in manufacturing such as, filtration, sieving, cyclone separation and the like.
LC polymerization is a term referring to a specific kind of suspension polymerization process. "Suspension polymerization" refers to a process in which a polymerizable liquid is dispersed as droplets in a continuous aqueous medium and polymerized under continuous agitation.
Normally, this process is carried out in the presence of a "granulating agent", such as a lyophilic polymer (starch, natural gums, polyvinyl alcohol or the like) or an insoluble fine powder such as calcium phosphate. These granulating agents help to obtain a dispersion of droplets of the polymerizable liquid but do not provide sufficient stabilization of the dispersion so that the dispersed droplets are stable in the absence of agitation. Therefore, in this method, it is necessary to carry out the polymerization under continuous mechanical agitation, since otherwise extensive coalescence of the droplets will occur, with separation of a bulk phase of the water immiscible, polymerizable material. Because this process depends on the details of the shear field in the reactor, and on the changing viscosity of the polymerizing dispersed phase, it is difficult to control reproducibly, is not readily scalable, and gives broad particle size distribution (PSD). In addition, conditions for one dispersed phase are not generalizable to other dispersed phases.
"Limited coalescence polymerization", also refers to a process in which the water-immiscible polymerizable liquid is dispersed in an aqueous medium. In distinction to the suspension polymerization method, however, the dispersion process is carried out in the presence of a colloidal water-insoluble particulate stabilizer which is capable of providing good stability to the dispersed droplets. In the presence of such a stabilizer, the droplets are stable once formed, even in the absence of mechanical agitation. The polymerization can therefore be carried out with no or minimal stirring (only enough to prevent creaming or sedimentation and provide good thermal transfer). Various stabilizing colloids are well-known in the art (for example, clays, colloidal silica, and latex particles, as described, for example, in U.S. Pat. Nos. 5,133,912; 4,965,131 and 2,932,629). This process has also been referred to as "quiescent suspension polymerization". It has substantial advantages over normal suspension polymerization, in that it gives reproducible behavior, relatively narrow PSD which are a function principally of the amount of particulate colloidal stabilizer used (and not a function of the mechanical agitation), is readily scalable, and allows high productivity because large concentrations of polymerizable materials can be suspended in the aqueous medium.
The size and PSD of the particles obtained by LC polymerization depends on the stability of the monomer droplets to particle growth during the polymerization process. While the particulate stabilizers useful in LC provide excellent protection against growth by a coalescence mechanism, growth by a diffusional mechanism is still possible, particularly for monomers which have appreciable water solubility.
Thus, it can be readily seen that there is a need for a process of preparing polymeric particles that are uniformly sized and have reduced populations of oversized particles.